Cyclin E, an activator of cyclin-dependent kinase 2 (Cdk2) is a positive activator of the G1-S phase transition in proliferating mammalian cells and therefore serves an important function during development and in replenishment of tissues. However, cyclin E is frequently overexpressed and deregulated in human malignancies, indicating that excessive or mistimed cyclin E-Cdk2 activation can promote oncogenesis. Yet little is known about the normal or pathological functions of cyclin E at the cellular or molecular level. This proposal seeks to investigate both functions of cyclin E through a variety of in vitro and in vivo approaches. The first aim of the proposal focuses on the roles of cyclin E in promoting entry of cells into S phase. Standard molecular and cell biological approaches will be employed. We have also embarked on a proteomic strategy to identify new proteins relevant to the Sphase promoting function of cyclin E. The second aim utilizes cell and molecular biological approaches to elucidate the mechanism(s) whereby cyclin E deregulation causes DNA damage and genomic instability, the most likely link to oncogenesis. In the third aim, cyclin E transgenic mouse carcinogenesis models are used to test the hypothesis of cyclin Emediated genomic instability in vivo. This is critical, as most prior work has been carried out in cell culture models, which have only a limited relevance to human cancer. Finally, we are investigating the mechanism of cyclin E turnover, since cyclin E becomes deregulated when this process fails. Interestingly, the pathway that targets cyclin E for ubiquitin-mediated proteolysis also targets several other oncoproteins, underscoring its importance for oncogenesis. It is hoped that insights gained from understanding these fundamental mechanisms of oncogenesis will lead to new approaches to cancer prevention and therapy.